Field guide
The Alberta aerial capture calendar
Most of what determines whether an aerial capture succeeds has nothing to do with the aircraft. It is whether the ground is bare, whether the leaves are on, how high the sun sits, and what the crop is doing. In Alberta those things move fast and the good windows are narrow.
Written for the Edmonton region at 53.5 degrees north. Foothills and northern sites run a couple of weeks behind on both ends, and the further north you go the more the daylight swings in both directions.
The four things that set the window
Snow
Snow erases the surface. Terrain work, volumes and contours all need bare ground, and around Edmonton the dependable snow free stretch runs roughly from the first week of May to the first week of October. Shoulder weeks on either side are a gamble that sometimes pays: a dry, windy late April strips the fields early, and a warm October can hold the window open for an extra fortnight. Snow is not useless, though. A fresh even blanket is a clean backdrop for finding heat, and structures read well against it.
Leaves
This is the one people miss. A camera sees the top of whatever is there, so between green up and leaf drop a photogrammetric pass over treed ground gives you the canopy, not the ground. That is a surface model, and for drainage or earthworks it is the wrong answer. Two leaf off windows open each year in the Edmonton area: from melt off in April until green up around the third week of May, and from leaf drop in early October until the snow arrives. Both are short, both are the best photogrammetry you will get all year over treed or brushy ground, and both fill up fast. Outside them, lidar is the answer, because it reaches through canopy in any month.
Sun angle
At this latitude the sun swings further than most people expect. At solar noon it reaches about 60 degrees above the horizon at midsummer and about 13 degrees at midwinter. That 13 degrees matters because shadow length is set by it. At 60 degrees a ten metre light standard throws a shadow under six metres. At 13 degrees the same pole throws more than forty. Long shadows swallow detail in an orthomosaic and confuse the matching that builds a 3D model, so mapping wants the sun above roughly 30 degrees. Around Edmonton the sun clears 30 degrees at midday from about the first week of March to the second week of October, and it holds there for a usable stretch of hours only from April through September. The same low winter sun that ruins a map makes a beautiful photograph.
Heat
Thermal work depends on the difference between a warm thing and its cooler surroundings, which means the hours before the sun starts loading heat into everything. We fly flat roof moisture scans between dawn and shortly after sunrise for exactly this reason, and thermal searches in the last hours before sunset or the first after dawn. Winter is the season for building envelope and heat loss work, when the gap between a heated interior and outside air is at its widest. An overcast day is a gift in any season, because without direct sun the ground stays even and contrast holds much longer.
The year at a glance
8 capture typesMonth by month
Edmonton region| Month | What the ground is doing | What this month is for |
|---|---|---|
| January | Deep cold, full snow cover, sun barely clearing 13 degrees at noon. Shortest working days of the year. | Building envelope and heat loss thermal, when the inside to outside gap is widest. Winter construction progress. Striking low light media over snow. |
| February | Still frozen, but the sun is climbing fast and days lengthen by nearly three minutes a day. | The rest of the envelope thermal season. A good month to book spring mapping before the queue forms. |
| March | Alberta's snowiest month on average, with melt starting late. Sun crosses 30 degrees at midday early in the month. | Planning and scoping. Roof claims from winter damage start moving. Late March can open early on a dry year. |
| April | Melt off, bare fields, no leaves yet. Runoff is visibly sitting where the ground actually falls. The windiest stretch of the year. | The best month of the year for bare earth terrain. Drainage and grading work, because you can see where water genuinely collects. Post winter roof inspections. |
| May | Reliably snow free from the first week. Seeding underway. Green up arrives around mid to late month and closes the leaf off window. | Pre seeding bare soil mapping. The tail of the spring terrain window. Construction baselines as the season opens. |
| June | Full canopy, longest days of the year at roughly seventeen hours, sun at its highest. | Construction progress, stockpile volumes, media at its best. Lidar for anything that needs ground under cover. Early stage crop multispectral. |
| July | Peak growth, canola in bloom, warm and settled. | The heart of the construction and aggregate season. Crop health at peak biomass. Long days make big sites achievable in one mobilisation. |
| August | The clearest skies of the year and the start of harvest. | The most reliable flying weather on the calendar. Progress documentation, volumes, media, and pre harvest crop work. |
| September | Harvest through the month, first frosts arriving, stubble replacing standing crop. | Post harvest field mapping with the ground newly visible. Drainage planning ahead of freeze up. Year end construction records. |
| October | Leaf drop through the first half, snow returning around the first week. Sun drops under 30 degrees near mid month. | The second leaf off window, and the last clean terrain capture of the year. Narrow, weather dependent, and worth booking ahead. |
| November | Snow settling in, sun low, shadows long. | Envelope thermal season opens. Winter site records and documentation before freeze up is complete. |
| December | Shortest days at about seven and a half hours, sun at 13 degrees and shadows more than four times object height. | Thermal work at first light. Media that uses the low sun deliberately. Planning for the April window, which fills earlier every year. |
Three things worth planning around
PracticalIf the deliverable is a terrain model, aim for April
Bare ground and leaf off overlap for only a few weeks, and that overlap is the highest quality photogrammetry Alberta offers. Everything after green up needs either lidar or an acceptance that you are modelling the canopy.
Book the October window in August
It lasts a couple of weeks, it depends on when the leaves go and when the snow comes, and every engineer with a year end deadline wants it. The ones who get it asked early.
Overcast is not a delay
For thermal work a grey sky is better than a clear one, and for mapping, flat even light removes the shadows that cause trouble. The days we move are the windy ones, not the cloudy ones.
Tell us the deliverable and the deadline and we will tell you which window it needs, and what it would take to hit the next one.
Plan a capture windowHow the regions differ
Across AlbertaCalgary and south run a week or two ahead in spring and hold the fall window slightly longer, with chinooks opening odd mid winter gaps that are genuinely usable. The foothills hold snow later and get more of it, so the spring window compresses. Grande Prairie and the north run behind in spring and ahead into winter, with a shorter overall season but dramatically longer June days. River valley and low ground anywhere frosts earlier and holds moisture longer than the land above it.
Common questions
Answered plainlyWhen is the best time of year for drone terrain mapping in Alberta?
April, in most years. Bare ground and leaf off overlap for a few weeks after melt and before green up, and that overlap is the highest quality photogrammetry the Alberta year offers. A second window opens in early October after leaf drop and before the snow returns.
Can drone mapping be done in winter?
Thermal work is at its best in winter, when the gap between a heated interior and outside air is widest. Terrain work needs bare ground, so it waits for melt off. Low winter sun also makes striking media imagery.
Why does sun angle matter for aerial mapping?
Shadow length is set by the sun's altitude. Around Edmonton the sun reaches about 60 degrees at midsummer noon and about 13 degrees at midwinter, where a ten metre pole throws a shadow over forty metres long. Long shadows swallow detail in an orthomosaic and confuse the matching that builds a 3D model, so mapping wants the sun above roughly 30 degrees.
Does cloud cover delay a flight?
Rarely. For thermal work an overcast sky is better than a clear one, and for mapping, flat even light removes the shadows that cause trouble. Wind is what moves a capture day, not cloud.
Why is lidar used instead of photogrammetry in summer?
Between green up and leaf drop a camera sees the canopy rather than the ground, which produces a surface model instead of a terrain model. Lidar returns ground points through vegetation in any month, so it carries the work through the seasons photogrammetry cannot.